Explanation:
Only a few short years ago, when the
APOD editors were in graduate school,
the pervasive,
cosmic
Dark Energy was not even seriously discussed.
Of course, it now
appears that this strange energy dominates
the cosmos (as well as lectures
on
cosmology) and provides a
repulsive force accelerating the large scale
expansion of the Universe.
In fact, recent
brightness
measurements of distant and
therefore ancient, stellar explosions or supernovae
indicate that the universal expansion began to
speed up in earnest four to six billion years ago,
when the Dark Energy's repulsive force began to
overcome the attractive force of gravity over cosmic distances.
The
Hubble Space telescope images above show a sample of the
distant supernova explosions, billions of light-years away, in before
(top) and after (bottom) pictures of their faint host galaxies.
Hubble
measured supernovae also hint that the
Dark Energy's
repulsive force is constant over cosmic time and so could be
consistent with Einstein's
original theory of gravitation.
If the force actually changes with time, the Universe
could still end in a
Big Crunch or a
Big Rip ... but not for at least an
estimated 30 billion years.